[Investigation of reduced vancomycin susceptibility in methicillin-resistant staphylococci]

The first Staphylococcus aureus strain with reduced susceptibility to vancomycin was reported from Japan in 1996, and since then an increasing numbers of cases had been reported from various countries. Along with the unfeasibility in the identification of these strains with routine laboratory methods, the use of glycopeptid antibiotics in infections due to these strains may result in therapeutic failure. The aim of this study was to investigate the prevalence of vancomycin intermediate staphylococcus (VIS) and heterogenous VIS (hVIS) strains with the use of agar screening, macro E-test, and population analysis profile (PAP-UC; population analysis profile-area under the curve) methods. A total of 148 methicillin-resistant staphylococcus strains isolated from different clinical samples (48 tracheal aspirate, 48 blood, 39 wound swabs, eight urine, two cerebrospinal fluid, two pleural fluid, one catheter tip sample) between November 2007 and May 2009, were included in the study. Of the isolates 107 were identified as S.aureus and 41 were coagulase-negative staphylococci (CNS; 23 Staphylococcus epidermidis, six Staphylococcus haemolyticus, five Staphylococcus chromogenes, three Staphylococcus hominis and four others) by API Staph kit (bioMerieux, USA). Methicillin resistance has been determined by standard disk diffusion method with oxacillin (1 µg) and cefoxitin (30 µg) disks, according to "Clinical and Laboratory Standarts Institute (CLSI)" guidelines. For the identification of VIS and hVIS strains, brain-heart infusion agar plates containing 6 µg/ml vancomycin (BHI-V6) were used for screening. The suspected VISA/hVISA strains which grew in this agar were further tested by macro E-test and PAP-AUC methods. Total VIS and hVIS rates among the tested isolates, were found as 3.4% (5/148) and 1.4% (2/148), respectively. These rates for CNS strains were 9.8% (4/41) and 2.4% (1/41), and for S.aureus strains were 0.9% (1/107) ve 0.9% (1/107), respectively. In the evaluation of the seven patients who were infected with VISA/hVISA strains, it was detected that all had history of use of glycopeptid antibiotics except one whose history was not reached, and all were hospitalized in intensive care units, except one who had an infected knee prosthesis. Since macro E-test and PAP-AUC methods could not be performed for all of the isolates, there was a probability that our resistance rates did not reflect the real results, nevertheless VIS and hVIS prevalence that we found in our study, seemed to be higher than those data reported previously from our country. In conclusion, since the number of VISA/hVISA strains may increase in time, surveillance for vancomycin resistance in methicillin-resistant staphylococci should be carried out in hospitals periodically.